Former South Korean "comfort woman," Kim Bok-dong, 87, front, who was forced to serve for the Japanese Army as a sexual slave during World War II, seen here in April.

By Arata Yamamoto, Producer, NBC News

TOKYO -- The outspoken mayor of Osaka is under fire not only from the government but from members of his own party for saying that the use of “comfort women,” some of whom were forced into prostitution, during World War II was necessary for the morale of Japanese soldiers.

Toru Hashimoto, co-leader of the Japan Restoration Party, made the comments during a news conference Monday.

“Whether it was of their own volition or against their will, the comfort women system was something necessary,” he said. “For military morale back then, it was probably necessary.”

The comments brought a quick backlash from senior Japanese politicians.

One of the strongest rebuttals came from a top official in Hashimoto’s own party.

“This is not something that’s coming out of our party. I think Mr. Hashimoto was expressing his own private opinions,” said Sakihiti Owaza, a senior official in the Japan Restoration Party. “If these comments continue, we will need to look into his true intentions and put a stop to this.”

Toru Yamanaka / AFP - Getty Images, file

Osaka Mayor and co-leader of the Japan Restoration Party Toru Hashimoto, seen here in 2012.

Yoshihide Suga, chief cabinet secretary in the government of Prime Minister Shinzo Abe, declined to directly criticize Hashimoto; doing so would be considered inappropriate because they are members of different parties.

He said, however, that the government’s position on the matter was clear: "The issue of comfort women is an experience of an unspeakable, painful suffering for which we also feel extreme anguish.”

Cabinet Minister Tomoko Inada did not let the protocols of political politeness stand in her way.

“It might not be appropriate to comment on what has been said by a leader of another party, but I believe the system of comfort women was a tremendous violation of women's human rights,” she said.

Education Minister Hakubun Shimomura said he heard about the comments while on visits to Washington and London and he thought they had been not been “properly understood” by foreign media.

Despite that, given the tensions between Japan and its Pacific neighbors, he said that “the timing of Mr. Hashimoto’s comments couldn’t have been worse.”

“I strongly wonder where there was anything positive in making these comments,” he said.

Hashimoto’s remarks about comfort women represented a break with what has become a Japanese tradition.

In 1994, then-Prime Minister Tomiichi Murayama issued an official apology for Japan's conduct before and during the war, including the treatment of those who came to be known as comfort women. Since then, subsequent administrations have upheld Murayama’s apology.

On Monday, Hashimoto agreed that it was important to accept Japan's role as an aggressor in the war and apologize for its atrocities, but he argued that other countries have had brothels for their troops.

"When a group of men is risking their lives, when this group of men are in a psychologically tense state, … anyone could understand that they would need something like the comfort women system," he said.

By Tuesday, there was evidence that Hashimoto might be stepping back a bit – but not retreating.

"Just because it was right at the time, obviously you cannot justify it today,” he wrote in a Twitter post.